1. Field of Invention
The present invention relates to a scanning electron microscope which includes a wide visual field mode (a mode for inspection of large area) and a high resolution mode (a mode for observation of high resolution).
2. Description of the Prior Art
Recently, a pattern size in a LSI is less than a wavelength for exposing it, below a design rule of 250 nm because of acceleration of miniaturization of the LSI. Therefore, ultra resolution techniques using proximity effect correction, phase shift mask, and so on have been required.
In process control under such circumstance, a conventional method for measuring one-dimensional length, which controls only width of line of the pattern, has not be useful any more. Therefore, two-dimensional control or three-dimensional control of the pattern for the process control has been demanded.
In the process control, throughput is an important factor. A process control which measures a large area of mask and resist pattern in a short time and compares the measurement with standard pattern and CAD data is required. In order to carry out this control, a scanning electron microscope capable of scanned a large area of 1000 μm to 2000 μm squares with the resolution of 20 nm to 50 nm is required.
Usually, one side of area of chip is 10 mm to 20 mm. In the pattern inspection, in order to improve the throughput, it is necessary to observe the area as large as possible without strain even though quality of resolution is lowered in some degree. Consequently, it is appropriate to use objective lens with long focal lengths.
On the other hands, when a defect is found in the mask and resist pattern, it is necessary to observe the part in detail with high resolution in order to analyze the defect and clarify the cause. In order to observe the defect, a scanning electron microscpe having the high resolution of the order of 3 nm to 5 nm, is required. Therefore, it is desirable to use an objective lens with short focal length for the inspection of defect.
However, it is difficult to scan the large area without the strain by the objective lens with short focal length. Therefore, these two requirements are incompatible.
Meanwhile, a conventional method for inspection of light using laser beam is practically much faster than electron beam, considering the throughput. However, the resolution of light is limited by wavelength.
Even in case of using ultraviolet rays, the resolution is limited on the scope of 50 nm to 100 nm under the present condition.
An advantage of using an electronic microscope is to achieve the resolution below the resolution of light, in other words, it is capable to achieve the resolution below the 50 nm.
In order to achieve the resolution below 50 nm, it is desirable for WD (working distance) to be below 50 nm to 70 nm of the resolution as shown in FIG. 5. Moreover, the resolution of 1 nm can be achieved by using the electron beam as clearly shown in FIG. 5. However, even though the electron beam is used, high resolution cannot be achieved if the distance (working distance (WD)) between the objective lens and a sample is long.
For example, the WD should be less than almost 7 mm as shown in FIG. 5, in order to maintain the resolution of 5 nm with one objective lens. Therefore, the objective lens should be as close as possible to the sample.
However, it is necessary to adjust dynamically each aberration such as astigmatism, curvature, distortion, and so on in cooperation with the scanned in order to scan wide field of 1 mm by the WD of 7 mm.
However, in this condition, because the distortion aberration reaches 1 mm, it is difficult to correct the distortion in a correction circuit. Moreover, a problem such as inability of correction of the distortion with the correction circuit is generated.
Furthermore, even though the distortion is corrected by force, operation and the adjustment become complicated as raising cost. Off-axis chromatic aberration, which is hard to correct, reaches close to 150 nm. On the other hands, the off-axis chromatic aberration becomes larger by making the WD longer, and as a result, the resolution of 5 nm cannot be maintained.
One objective lens of the scanning electron microscope to inspect the pattern on wafer printed from the mask cannot cover the incompatible functions such as large area inspection function and high resolution observation function.
Moreover, FIG. 5 is a graph showing relation between the resolution when the electric potential E of sample is −2.5 kV with 3 kV of acceleration voltage and distance from the sample to center of the objective lens.